1. Field of the Invention
The present invention relates to oligomerizing ethylene, in particular its trimerization, principally to 1-hexene.
2. Description of the Prior Art
Processes for producing alpha-olefins from ethylene generally result in an assembly of oligomers containing 4 to 30 carbon atoms or even more, the olefins obtained then being separated by distillation. For several years, demand for lower oligomers has been increasing, essentially for 1-butene, 1-hexene and 1-octene, which are used in particular as co-monomers with ethylene in producing linear low-density polyethylene.
Few catalysts exist that selectively result in the formation of a particular oligomer, as is the case when dimerizing ethylene to 1-butene with a titanium-based catalyst. However, it is known that catalysts based on chromium can principally result in the formation of 1-hexene, with greater or lesser quantities of polyethylene, the proportion of butenes and octenes in the products being very small (R M Manyik, W E Walker, T P Wilson, J. Catal., 1977, 47, 197 and J R Briggs, J. Chem. Soc., Chem. Commun. 1989, 674 and cited references). Catalysts that allow more or less selective trimerization of ethylene have been described, for example, in U.S. Pat. Nos. 5,198,563, 5,288,823, 5,382,738 and European patent applications Nos. 608,447, 611,743 and 614,865. These catalysts are prepared from a chromium salt and a metallic amide, in particular a pyrrolide. Other catalysts make use of an aluminoxane and a chromium complex with a chelating phosphine (U.S. Pat. No. 5,550,305).
French Patent No. 2,764,524 describes a catalytic composition obtained by mixing at least one chromium compound with at least one aryloxy aluminum compound and at least one hydrocarbylaluminum compound, which has a particular selectivity for forming 1-butene and/or 1-hexene by oligomerizing ethylene.
French patent application No. 2,802,833 describes a catalytic composition obtained by mixing at least one chromium compound with at least one aryloxy compound of an element selected from the group formed by magnesium, calcium, strontium and barium, and at least one hydrocarbylaluminum compound, which has a particular selectivity for forming 1-hexene by oligomerizing ethylene.